1. Field of the Invention
The present invention relates to an apparatus for the automatic adjustment of the gap between printing head and platen in a serial impact printer.
2. The Prior Art
It is known that the impact serial printers, generally dot matrix printers, are ubiquitous on the market as computer peripheral units.
Such printers may print on printing media having differing thickness, such as single sheets, multiple-copy media, books.
Depending on the media thickness they need an adjustment of the gap between the printing head and platen where the printing media is positioned. This adjustment can be performed manually by means of suitable adjusting devices or automatically.
Mechanical adjustment devices have long been proposed. An example of such devices may be found in U.S. Pat. No. 3,990,560.
Mechanical devices do not provide the reliability required in modern printers.
Recently impact printers have appeared on the market which are provided with more reliable automatic gap adjustment devices.
In such printers, the printing head is slidably mounted on a guiding shaft parallel to the platen. The shaft is pivoted at its ends within two mounting bushings and can rotate in such bushings. The bushings are fixed on the side plates of the printer frame at a predetermined distance from the platen.
The ends of the shaft, pivoted within the bushings, are eccentric as to the shaft so that by changing the angular position of the shaft its distance from the platen and therefore the distance of the printing head from the platen is changed. The shaft is rotated by a motor (generally a step motor) controlled by logic circuits.
A conductive rubber pad, whose resistance depends on the pressure exerted on it, is used as position detector. The rubber pad is mounted on the printing head, close to the printing elements. When the print head is correctly positioned as to the printing media and the platen, the pad is in contact with the media, slightly compressed by the media, and provides a corresponding current/voltage electrical signal when suitably powered by a voltage/current generator.
This signal, whose amplitude depends on the contact pressure, controls, through suitable circuits, the motor which rotates the shaft, so as to impose a distance of the printing head from the media such that the signal generated by the position detector has a predetermined amplitude, corresponding to a correct positioning.
Although the detector element is particularly simple and inexpensive, it requires amplifying circuits, temperature compensation circuits and trimmers which are rather expensive.
Moreover, the electrical characteristics of the detector are subject to drift over time, owing to aging of the material and temperature changes.
Therefore, the resulting apparatus is rather expensive and at long term unreliable, unless frequently trimmed.
These disadvantages are overcome by the apparatus for the automatic adjustment of the gap between a print head and a printing media which is the object of the present invention.
This apparatus is reliable, stable in operation, very simple and inexpensive.